Be sure to measure your specific use-case before jumping
to conclusions.
With our application, for example, Btrfs performed exceptionally
bad - about 4 times(!) as slow as XFS.
Then again, there are some use-cases where even older
file-systems like reiser3 excel (e.g. storing files for
cyrus imapd).

But I'm stunned that XFS is that much slower than ext4 in many tests.

Again, it all depends on the use-case. For us, ext4
performs good (when used with all kinds of performance-enhancing,
safety-reducing mount-options), but not as good as XFS.
To me, as of today, XFS' big strength is performing good to
excellent (while not always better than all other file-systems)
in many use-cases - without worries about instability or immaturity.
One thing, I guess, is for sure: Every file-system will require
continued development to stay competitive.
SSDs, for example, are just beginning to get used appropriately
by modern file-systems. There's plenty of opportunity left to
optimize for them.
And once that is done, there may be yet another storage-technology
available (PRAM? Racetrack?), that benefits from specific strategies.
So the competition will stay open... :-)
Regards,
Peter Niemayer